The present invention relates to a fuel cell system and a method of discharging gas from the fuel cell system.
In recent years, developments have been promoted on fuel cell vehicles which are equipped with a fuel cell such as a PEFC (Polymer Electrolyte Fuel Cell). The fuel cell generates electricity when hydrogen is supplied to the anode and oxygen is supplied to the cathode. The fuel cell vehicle runs when electric power generated by the fuel cell rotates the motor.
In such a fuel cell, hydrogen is generally supplied to the anode for an amount more than required for hydrogen consumption in terms of improved output of the fuel cell. For this reason, the unused remaining hydrogen is discharged from the anode of the fuel cell. To improve the usability of hydrogen, a hydrogen circulating system has been adopted for returning the discharged unused hydrogen to the hydrogen supply side to thereby circulate the hydrogen.
When such a fuel cell generates electricity, water is generated at the cathode. A part of this generated water then permeates through the polymer electrolyte membrane (hereinafter referred to as an electrolyte membrane) and flows to the anode. In order to ensure humidity of the electrolyte membrane and thus to improve the diffusion (conductivity) of protons (hydrogen ions) of the electrolyte membrane, there has been proposed a method in which a reaction gas (air containing hydrogen and oxygen) which is supplied to the cathode or the anode of the fuel cell is humidified.
In the case of a fuel cell system using the hydrogen circulating system, the amount of water accompanying the circulated hydrogen increases as the advancement of generation of electricity, which may lead to a decrease in the power generation efficiency of the fuel cell. For this reason, according to Japanese Laid-open Patent Application No. 2000-243417 (see paragraph number [0022] and FIG. 1), the circulated hydrogen-containing gas is discharged at predetermined intervals, which is called as purging, so that the power generation efficiency of the fuel cell can be recovered. Since the discharged gas contains hydrogen, dilution gas (cathode off-gas) for diluting the discharged gas is introduced into an exhaust fuel diluter together with the discharged gas so that the discharged gas is diluted by the dilution gas and then emitted into the atmosphere.
However, the fuel cell vehicle runs on a highland where the atmospheric pressure is high and on a lowland where the atmospheric pressure is low. Therefore, according to the change in the atmospheric pressure, the pressure of the purge valve, by which gas is discharged from the hydrogen circulating system, changes at the downstream side.
If the purge valve is opened, regardless of the change in the atmospheric pressure, at a certain time interval to discharge the gas from the hydrogen circulating system, the amount of discharged gas to be discharged at a time when the purge valve is opened may be different because of the change in the atmospheric pressure. For example, when the vehicle runs under a low atmospheric pressure, the amount of discharged gas containing hydrogen exceeds the setting amount. This may cause the gas to be emitted into the atmosphere without being diluted sufficiently by the exhaust fuel diluter. On the contrary, when the vehicle runs under a high atmospheric pressure, the amount of discharged gas containing hydrogen lowers the setting amount. This may cause a delay in recovery of the power generation efficiency of the fuel cell.
With the foregoing drawbacks of the conventional fuel cell vehicles in view, the present invention seeks to provide a fuel cell system which can discharge gas in accordance with a change in an atmospheric pressure and a method of discharging gas from the fuel cell system.